1) Field of the Invention
The present invention relates to a method and apparatus for detecting an abnormal state in a pulse train generating sensor. Such a pulse train generating sensor can be used, for example, for measuring the flow rate of engine intake air.
2) Description of the Related Art
In order to electronically control an internal combustion engine at an optimum condition, sensors for accurately detecting a driving state, memory devices for storing data and programs, and a microprocessor and actuators for the fuel injection, the ignition timing, and the like are provided.
Among these elements, the sensors are very important and detection of an abnormal state of the sensors must be made at an early stage. For this purpose, various self diagnosis devices for detecting an abnormal state in the sensors have been proposed.
Note, the output characteristics of conventional sensors for measuring a driving state of the engine, especially sensors for measuring a load of the engine, are linear. In an abnormal state, the output voltages of such sensors are usually 0 V (ground voltage) or V.sub.cc (equal to the supply voltage). Thus, such an abnormal state can be detected by determining whether the output voltage of the sensors is 0 V or V.sub.cc.
However, for pulse train generating sensors, for example, Karman vortex sensors which detect an intake air flow rate to the engine, it is impossible to detect an abnormal state by detecting the output voltage, since the outputs thereof are usually 0 V or V.sub.cc. Accordingly, the above-mentioned diagnosis for detecting an abnormal state cannot be applied to pulse train generating sensors.